Method and apparatus for determining concentration of NH-containing species

ABSTRACT

A method and apparatus for determining ammoniacal species concentration in a gas sample. In one embodiment, trace concentration of ammonia in an air sample is determined by monitoring emission intensity from an excited radical species (NH*), which is produced in a reaction between ammonia and fluorine. The observed emission intensity is compared with calibration data obtained from previously analyzed gas samples containing ammonia. The method and apparatus can also be adapted to detect ammoniacal species concentration in other NH-containing gas samples.

CROSS REFERENCES AND RELATED APPLICATION

[0001] This application is a divisional of co-pending U.S. patentapplication Ser. No. 09/624,118, filed Jul. 24, 2000, which also claimspriority to U.S. provisional patent application Serial No. 60/147,017,entitled “Method and Apparatus for Determining Concentration ofNH-Containing Species,” filed on Aug. 3, 1999, which are hereinincorporated by reference.

BACKGROUND OF THE DISCLOSURE

[0002] 1. Field of the Invention

[0003] The invention relates generally to a method and apparatus ofdetermining gas phase species concentration, and more particularly, to amethod and apparatus for detecting concentration of NH-containingspecies.

[0004] 2. Description of the Background Art

[0005] The use of ammonia (NH₃), a corrosive and toxic gas, inindustrial processes is wide spread. Trace amount of NH₃ has also beenshown to adversely impact the use of chemically activated deepultraviolet photoresists in advanced semiconductor fabrication. The needfor worker protection, from either acute exposure to high NH₃concentrations or long term exposure to very low concentration levels,has resulted in the development of sampling methods for the detectionand quantitative measurement of NH₃ in ambient air. Some existinganalytical techniques for NH₃ detection are briefly described below.

[0006] a. Electrochemical Method

[0007] In this method, gaseous NH₃ is absorbed into an electrochemicalsensor assembly with a resultant change in the electrical conductivityof the sensor cell. The increased current flow allowed by the sensor isfairly linear over the concentration range of 1-50 ppm. A lowerdetection limit is about 500 ppb, but reproducibility of the sensor toperiodic exposure of NH₃ is only fair.

[0008] b. Ozone Method

[0009] This method uses a reaction between ozone (O₃) and ammonia, inwhich NH₃ is first converted to NO₂, followed by a chemiluminescentreaction between NO₂ and O₃. The reaction with O₃ results in theformation of excited state NO₂ molecules, denoted as NO₂*, and theintensity of emission from NO₂* is used to determine the original NH₃concentration. However, difficulties in quantitative measurement resultfrom side reactions during the conversion from NH₃ to oxides of nitrogen(forming NO and, perhaps, NO₃ or HNO), and also from non-stoichiometricside reactions between NO₂ and O₃. In addition, the emission fromexcited NO₂ species (NO₂*—the asterisk “*” is used in this disclosure todesignate an excited state of a species) extends from the near UV intothe yellow-green region of the visible spectrum (this emission is thewell known “air afterglow” in the night sky, and results from thereaction: NO+O₂→NO₂*+O). Detection of this very diffuse emission over abroad spectral region is susceptible to interference from other emittingspecies, and may pose difficulties in accurate concentrationdetermination.

[0010] c. Air Sampling Method

[0011] In this method, air samples are collected via a carefullyprepared evacuated sampling ampoule and injected into a gaschromatograph (GC) for comparison against analyzed standards by wellknown methods. Careful selection of the GC column and temperaturesettings are necessary in order to obtain reliable results. A number ofdetectors are available for this method. One very sensitive detectionmethod is mass spectrometry, but calibration for quantitative work isvery difficult. Additionally, the GC/MS method is very expensive, and itis difficult to configure in a continuous sampling mode.

[0012] d. Laser Induced Emission

[0013] This method has the potential for great sensitivity, but requiresgreat expertise and expense due to its sophistication. NH₃, or afragment thereof, is electronically (or vibrationally) excited by apulsed, tunable dye laser, thereby creating observable fluorescence.However, non-linear optical effects and saturation effects tend to makequantitative measurements extremely complex, if at all possible.

[0014] Each of these prior art techniques has its own limitation andvarying degrees of experimental complexities. Therefore, a need existsin the art for alternative analytical methods that allow continuouson-line determination of low level of ammonia in ambient air or gassamples.

SUMMARY OF THE INVENTION

[0015] Embodiments of the invention generally provide a method andapparatus for determining the concentration of an NH-containing speciesin a gas sample. The method comprises detecting radiation from excitedimidogen radicals (NH*) generated from the gas sample, and determiningthe concentration of the NH-containing species from calibration datacorrelating detected NH* radiation intensity with concentration of theNH-containing species. In one embodiment, the NH-containing species isammonia (NH₃), and the NH* radiation is generated by reacting NH₃ with agas sample containing fluorine. Using a bandpass optical device, NH*radiation around 336° nm can be selectively transmitted and detected,with minimal interference from other emitting species.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016] The teachings of the present invention can readily be understoodby considering the following detailed description in conjunction withthe accompanying drawings, in which:

[0017]FIG. 1 depicts a schematic diagram of an apparatus for determiningammonia concentration according to one embodiment of the invention; and

[0018]FIG. 2 is an illustration of a calibration plot that can be usedfor determining concentration of ammoniacal species.

[0019] To facilitate understanding, identical reference numerals havebeen used, where possible, to designate identical elements that arecommon to the figures.

DETAILED DESCRIPTION

[0020] The present invention generally provides a method and apparatusfor determining concentration of an ammoniacal (ammonia-like, orNH-containing) species in a gas sample. In one embodiment, theammoniacal species is ammonia (NH₃). It is known to those skilled in theart of molecular spectroscopy that gaseous NH₃ and molecular fluorine(F₂) will spontaneously react, typically at sub-atmospheric pressures.As is the case with many gas phase reactions, several chemical reactionpathways are possible, giving rise to different reactive or non-reactiveintermediate or product species. It is also known that light emissionaccompanies this spontaneous reaction, and that the emission ischaracteristic of energetic, or excited state, species generated in thereaction. Among these electronically excited species is the diatomicimidogen free radical (NH*), which has a spectral emission in anunusually narrow wavelength region around 336° nm (due to the NHA³Π-X³Σ⁻ transition). This light emission, also known as fluorescence,is the predominant emission in the visible and ultra-violet (UV) regionof the optical spectrum from the spontaneous reaction between NH₃ andF₂. When the emission is generated from a chemical reaction, it issometimes referred generally as chemiluminescence.

[0021] Embodiments of the invention provide a method and apparatus bywhich a trace concentration of NH₃ can be determined from a functionalrelationship between the NH₃ concentration and the observed NH* emissionintensity, where NH* is used to denote generally an excited state of theNH species. In particular, the method relies on two assumptions: 1) thatthe detected NH* emission intensity (I_(NH)*) is proportional to theconcentration of NH* species; and 2) that the concentration of NH* is inturn correlated with the initial NH₃ concentration prior to the reactionwith F₂.

[0022] The first assumption can be expressed as:

I _(NH) *°=°k*[NH*]  Eq.(1);

[0023] where k* is a proportionality constant related to a variety offactors specific to the experimental setup, including light collectionefficiency, detector sensitivity, and the like; and [NH*] is theconcentration of excited NH species present in the detection volume.

[0024] The second assumption can be expressed as:

[NH*]°=°k ₁ f([NH ₃])  Eq.(2);

[0025] where k₁ is a proportionality constant, and f([NH₃]) denotesgenerally a function of the concentration of NH₃. Again, k₁ is anexperimental constant which depends on a variety of factors related tothe reaction kinetics between NH₃ and F₂. This, along with Eq. (1)above, leads to:

I _(NH) *=kf([NH ₃])  Eq.(3);

[0026] where k=k*k₁.

[0027] According to the method of the invention, the concentration ofNH₃ present in a gaseous sample can be determined by experimentallymeasuring the intensity of emission from NH*, and determining the NH₃concentration [NH₃] from the functional relationship of Eq.(3). Theexact functional relationship f([NH₃]) can be obtained by a calibrationprocedure to be described below. The method is particularly suited tothe determination of trace level of NH₃ in a gas sample.

[0028] In general, the concentration of an intermediate species in areaction, such as an excited state of a reactive radical (NH*), is verylow, and one may encounter difficulties in detecting emission from sucha species. However, one can take advantage of the fact that thepredominant visible and UV emission from the NH₃+F₂ reaction originatesfrom NH*. By using a suitable bandpass optical device, such as anoptical interference filter or monochromator, one can selectivelytransmit and detect the NH* emission around 336° nm to the exclusion ofbackground signal from other emitting species. Any background emission,if not properly excluded, may interfere with (i.e., contribute to) theobserved light emission intensity and thus affect the accuracy of thedetermination of NH₃ concentration.

[0029] Since the reaction between NH₃ and F₂ occurs in the absence ofheating or other external energy sources (i.e., as a “dark reaction”),the resulting fluorescence can be measured against a dark background.This allows the use of extremely sensitive light detection methods, suchas photon-counting, to detect and quantify trace amounts of NH₃ presentin a gas sample, such as an ambient air sample containing NH₃. Hence,the invention has superior sensitivity over existing methods.

[0030] An apparatus suitable for practicing the present invention isillustrated schematically in FIG. 1. The apparatus 10 comprises a vacuumsystem 100 and an optical detection system 160. The vacuum system 100further comprises a reaction vessel 102 (or reactor) connected to apressure-reducing device such as a vacuum pump 180 and other gas flowand pressure regulating components. As shown in FIG. 1, the gas flow andpressure regulating components may illustratively comprise vacuum valves104, 106, 108 and 110. The valve 104 controls gas flow at the inlet 124,while the valve 108 controls gas flow at the outlet 128. At least one ofthese valves 104, 108 should have an adjustable orifice for variable gasflow control, such as that provided by a needle valve. Different needlevalves with varying sizes of orifices can result in a fine control ofthe gas flow up to a range of, for example, 500° sccm. The exact flowrange, however, is not critical to the practice of the presentinvention.

[0031] The valve 110 is a throttle valve connecting the outlet 128 tothe vacuum pump 180 via a vacuum line 184. For example, the vacuum pump180 may be a mechanical pump with inert fluorocarbon oil having a 2 CFMpumping capacity. The exhaust gases, including carrier gas, reactant andproduct gases, are evacuated through the vacuum line 184. The pumpingcapacity (or speed) provided to the vessel 102 can be varied byadjusting the throttle valve 110. The adjustment of valves 104 and 108,in conjunction with the throttle valve 110, allows control of the gasflows through the vessel 102. Thus, a partial vacuum in the range ofabout 0.1° mbar to about 50° mbar can be achieved inside the vessel 102.A pressure transducer 182 is also provided for pressure measurement. Itis preferable that more than one pressure gauge be used for pressuremonitoring at different pressure ranges. For example, capacitancemanometers available from MKS Instruments, Inc., Andover, Mass., aresuitable for this purpose.

[0032] The reaction vessel 102 also comprises a second inlet 126. Avalve 106 is used to control the gas flow through the inlet 126, whichextends into the interior 102I of the vessel 102, and terminates in aninlet tip 127. An optical window 152 is provided on one side 103 of thevessel 102 at close proximity to the inlet tip 127. The reaction vessel102 is preferably made of glass or quartz, but other materials such asstainless steel are also acceptable, as long as it is compatible withthe chemicals or gases used. The optical window 152 should be made of amaterial which can transmit radiation around 336° nm. In general, anyultra-violet (UV) transmitting materials such as different grades ofquartz will suffice.

[0033] To perform the NH₃ concentration measurement according toembodiments of the invention, the vessel 102 is evacuated with thethrottle valve 110 and valve 108 fully open. After a base pressure ofabout 0.1° mbar or below is reached, the valve 108 is closed to someappropriate intermediate position while a gas sample to be analyzed,e.g., air containing an unknown concentration of NH₃, is introduced intothe vessel 102 via the inlet 124. The flow rate of this NH₃/air samplethrough the vessel 102 can be controlled by adjusting the valves 104 and108. Asteady flow of the gas sample may be established within a range ofabout 100-500° sccm, preferably at about 300° sccm. An operatingpressure in the range of about 0.1° mbar to about 50° mbar, preferablyat about 10° mbar, may be used.

[0034] With the NH₃/air flow rate and pressure established, a second gassample containing fluorine—e.g., a dilute mixture of F₂ in a carrier gassuch as argon (Ar) or helium (He), is then is introduced into the vessel102 through the inlet 126 by the valve 106. This fluorine-containing gassample, also referred to as a reactant gas, is used to initiate areaction between NH₃ and F₂. The reactant gas is preferably a highlydiluted mixture of F₂ in a non-reactive carrier gas such as ultra-highpurity (UHP) Ar or UHP He. Of course, other similarly non-reactive orinert gases, e.g., nitrogen (N₂), may also be used as a carrier gas,provided that they do not substantially interfere either with the NH₃+F₂reaction or the detection of the NH* emission. The F₂/carrier gas mixeswith and reacts with the flow of air containing NH₃ (or generally, thegas sample to be analyzed) just down-stream of the gas inlet tip 127.This counter-flow reaction method and apparatus design is well known inexperimental gas kinetics.

[0035] A reaction zone 150, where F₂ and NH₃ reaction occurs, isgenerally defined in the vicinity of the reactant gas inlet tip 127inside the vessel 102. By controlling the flow rate of the F₂ gas intothe air/NH₃ flow stream, one can confine the reaction zone 150 to withina relatively small, well-defined volume. A better defined reaction zone150 is preferable because it allows an efficient collection anddetection of the chemiluminescence.

[0036] As the reactant gas reacts with NH₃ in the NH₃/air sample,emission from excited NH* species is detected using the opticaldetection system 160 to be described below. The reactant gas flow shouldbe adjusted so as to maximize the NH* emission intensity I_(NH)*detected by the optical detection system 160. That is, at a fixed flowrate of NH₃/air, the reactant gas flow should be sufficiently high suchthat additional F₂ (or reactant gas) will not result in an increase ofdetected I_(NH)* signal for a given configuration of the opticaldetection system 160.

[0037] It is understood that the process parameters disclosed herein aremeant to be illustrative, and other gas flow rates and operatingpressures may be adjusted as appropriate to different reaction vessels.In general, the choice of the operating pressure may be based on severalconsiderations—e.g., a higher operating pressure tends to favor a higherreaction rate between NH₃ and F₂. However, a higher pressure alsoresults in increased collisions between the excited NH* and other gasmolecules. These collisions may lead to “quenching” of the NH* emission,and thereby reduce the amount of detectable optical signal. Therefore,an optimal operating pressure may involve balancing these competingconsiderations, and one can experimentally arrive at the desiredoperating pressure by establishing initial flows of the NH₃ and F₂gases, and adjusting valves 104, 106, and 108 to maximize the NH*signal. Such optimization technique is well-known to one skilled in theart of chemical kinetics.

[0038] If the gas sample to be analyzed is being used as a process gasin a certain process application, the apparatus 10 may also be used forcontinuous on-line measurement of NH₃ concentration in the process gas.For example, the apparatus 10 may be connected (e.g., at its inlet 124)to a reactor (not shown) used for the particular process application,and a relatively small flow of the process gas may be diverted from thereactor into the reaction vessel 102 via the inlet 124. The NH₃concentration may then be continuously monitored according toembodiments of the invention, without interfering with the particularprocess application.

[0039] Optical Detection System

[0040] The light emission from the reaction of NH₃ and F₂ (due to the NHA³Π-X³Σ⁻. transition), is transmitted through a suitable optical window152 and a bandpass optical device 162, and detected by a detector 164. Alens 161, or similar imaging optics, may also be used to facilitate thecollection and imaging of light emission from a sample volume (e.g., thereaction zone 150) onto the detector 164.

[0041] The bandpass optical device 162 preferably has a bandpass that issufficiently narrow as to transmit the NH* emission near 336° nm, whilesubstantially rejecting emissions from other species that may interferewith the detection of the NH* emission (i.e., selectively transmittingthe desired NH* emission). In one embodiment of the invention, a narrowbandpass filter 162, e.g., an interference filter having a bandpass ofabout 10° nm (i.e., full-width bandpass at half-maximum intensity, orFWHM), with a peak transmission of about 10-50% around 336° nm may beused. Due to the “piling-up” of the Q-branch of the NH A³Π-X³Σ⁻electronic transition, most of the NH* chemiluminescence can betransmitted through the interference filter 162, which also effectivelyblocks other undesirable or background emissions, thus facilitating thedetection of NH* emission. Such an interference filter is available fromcommercial optics supply vendors. The optical characteristics of theinterference filter cited herein are meant to be illustrative. It isunderstood that filters with different optical characteristics (i.e.,FWHM bandpass, peak transmission percent and peak wavelength) may alsobe used to transmit the NH* emission for practicing embodiments of theinvention. For example, if measures are taken to eliminate interferingemissions (e.g., by eliminating species having interfering emissions), awider bandpass filter may be tolerated.

[0042] In other embodiments, the bandpass optical device 162 maycomprise a combination of different filters that is effective forselectively transmitting the desired NH* emission, while blockinginterfering emissions from other species. For example, the combinationmay include a longpass filter and a shortpass filter with appropriatecut-off wavelengths, or a bandpass filter having a FWHM bandpass largerthan about 10° nm and a suitable cut-off filter. One example of apossible interfering emission originates from OH radicals, which mayarise from the presence of moisture or other reactions in the reactor.It is known that an excited state of the OH radical has a strongemission around 306° nm. If a short wavelength cut-off filter (orlongpass filter) is used to block the 306 emission from excited OHradicals, then a filter having a FWHM bandpass larger than about 10° nmmay be used. Other bandpass optical devices such as a monochromator orsimilar equipment with wavelength selection capabilities can also beused in place of an interference filter.

[0043] The light emission that passes through the bandpass device orfilter 162 is incident upon the detector 164, which is selected to besensitive to the transmitted emission. For example, the detector 164 maybe a RCA 1P28 photomultiplier tube operating at about 800V DC. Thephotocurrent generated by the emission can be detected usingcommercially available detection and amplification equipment 166.Suitable detection and amplification equipment 166 may includepicoammeters or photon-counting devices with dynode pulse discriminationelectronics, among others. In general, various combinations of detectorsand amplification equipment may be used to detect the emission throughthe optical device 162 and convert it to a radiation intensity parameterthat correlates with the intensity of the NH* emission. The apparatus 10should preferably include a device 168 for monitoring and/or recordingof the amplified optical signal, or more generally, the radiationintensity parameter. The device 168 may illustratively be a computerthat interfaces with the detection and amplification equipment 166 andprovides for data storage and retrieval.

[0044] Calibration of the apparatus 10 is accomplished with dilute,analyzed samples containing NH₃—e.g., NH₃ in N₂ or in air, or othersuitable carrier gases. A calibration plot, for example, is constructedby plotting the chemiluminescent intensity during reaction with excessF₂ against known NH₃ concentrations [NH₃] from analyzed, calibration gassamples.

[0045] Calibration Procedure

[0046] In order to determine the concentration of NH₃ in an unknown gassample, a calibration procedure is performed in the reaction vessel 102to generate calibration data which correlate detected NH* emissionintensities with known NH₃ concentrations in calibration gas samples.The calibration gas samples, e.g., NH₃/air mixtures, can be analyzed toobtain known NH₃ concentrations by conventional analytical methods, orprepared by successive dilutions from more concentrated mixtures thatare amenable to conventional analytical techniques, or procured asanalyzed mixtures from any number of industrial gas suppliers.

[0047] The calibration procedure involves experimentally measuring theNH* emission intensities (I_(NH)*) from reaction with F₂ for severalcalibration gas samples with known NH₃ concentrations [NH₃], using theprocedure previously described. For example, NH* emission measurementscan be performed for each of several calibration gas samples containingNH₃ concentrations between a few hundred to a few thousand parts perbillion (ppb) by mixing each of the calibration gas samples with areactant gas containing fluorine. Although the fluorine-containingreactant gas used in the calibration procedure may be different fromthat used in the reaction with the gas sample having the unknown NH₃concentration, it is preferable and more convenient to use the samereactant gas (e.g., similar F₂ concentration and/or carrier gas).

[0048] The calibration data comprising detected emission intensityI_(NH)* (or a calibrated radiation intensity parameter correlating withI_(NH)*) and its corresponding NH₃ concentration [NH₃] may berepresented in a calibration plot, such as that illustrated in FIG. 2,which can be extrapolated to lower concentrations. More generally, afunctional relationship between I_(NH)* and [NH₃] can be derived fromthe calibration data. The NH₃ concentration in a gas sample with anunknown [NH₃] can then be determined by comparing the observed NH*emission intensity (from a reaction between the NH₃-containing gassample and the reactant gas containing fluorine) with the calibrationdata, or from the functional relationship correlating NH* emissionintensity I_(NH)* with NH₃ concentration.

[0049] In one illustrative embodiment, a mixture of 0.5% F₂ in UHP Arcan be used as a reactant gas for calibration. In practice, it is notpossible to maintain a stable concentration of highly diluted F₂ gas ina vessel (gas cylinder) for an extended period of time due to thecorrosive or reactive nature of F₂. However, this will not affect thecalibration procedure because F₂ is introduced in “excess” to produce amaximum detected I_(NH)* for the given detection system 160. Thereaction vessel 102 is first evacuated to a pressure of about 0.1° mbar.After a steady flow of one calibration NH₃/air sample (i.e., previouslyanalyzed, with known NH₃ concentration) is established, e.g., at apressure of about 10° mbar and a flow rate of about 300° sccm (orgenerally, the same pressure and flow rate as used for the reaction withthe gas sample having unknown NH₃ concentration), the F₂/Ar reactantmixture is introduced into the vessel 102 at the inlet tip 127. Toensure that F₂ is in “excess”, the F₂/Ar reactant gas is introduceduntil the observed NH* emission intensity no longer increases withadditional F₂/Ar reactant mixture. The NH* emission from the reactionzone 150 is detected as described above using detector 164.

[0050] The observed intensity I_(NH)*, along with the knownconcentration of the previously analyzed NH₃/air sample, may be recordedor stored in a suitable medium, e.g., a computer. The calibrationprocedure is repeated for the remainder of the calibration gas samples(that have previously been analyzed to obtain known NH₃ concentrations).A calibration plot such as that shown in FIG. 2 can be recorded, showingthe observed intensity I_(NH)* vs. the known NH₃ concentration. Ingeneral, a functional relationship derived from the calibration datawill be used for the determination of NH₃ concentration in gas samples.The calibration results should ideally be recorded by electronic means,for example, a computer or processor having a storage device, tofacilitate data storage and retrieval.

[0051] Other NH-Containing Samples

[0052] Although the present embodiment focuses on the measurement of NH₃in a gas sample, this invention can be extended to the determination ofother ammoniacal, or ammonia-like species, e.g., molecular species witha N-H bond, such as organic amines, imines and other NH-containingspecies. Although the detailed chemical reactions may differ, it isanticipated that reactions between different ammoniacal species andfluorine (contained in the reactant gas sample) will lead to theformation of excited NH species, or NH*. Depending on the specificammoniacal species, the reaction may not be “dark”, as previouslyexplained for the case of NH₃. However, the use of a narrowbandinterference filter should still suffice to isolate the emittedradiation from NH*, for example, around 336° nm, to allow for adetermination of the ammoniacal species concentration. Of course, aseparate calibration procedure has to be performed as previouslydescribed for a number of the gas samples containing knownconcentrations of the species of interest. The choice of pressure andflow parameters used in the F₂ reaction can readily be arrived atthrough experimentation which is known to those skilled in the art ofchemical kinetics.

[0053] Although one embodiment which incorporate the teachings of thepresent invention has been shown and described in detail herein, thoseskilled in the art can readily devise many other varied embodiments thatstill incorporate these teachings.

What is claimed is:
 1. An apparatus for determining concentration of anammoniacal species in a first gas sample, the apparatus comprising: areactor having a first inlet for introducing said first gas sample intoa reaction zone inside said reactor; an optical detection-system todetect radiation arising from said first gas sample inside said reactor;a second inlet for introducing a second gas sample containing fluorineinto said reactor, wherein said radiation arising from said first gassample is generated from a reaction between said first gas sample andsaid second gas sample within the reaction zone inside said reactor; anda data acquisition and storage system to convert said detected radiationinto a radiation intensity parameter, wherein said radiation intensityparameter is used to determine concentration of said ammoniacal speciesin said first gas sample.
 2. The apparatus of claim 1, wherein saidreactor further comprises a pressure measuring device, first and secondgas flow controllers to control said first and second gas sample flowsinto said reactor and a vacuum pump, wherein said pressure measuringdevice, said first and second gas flow controllers and said vacuum pumpcooperate with each other to maximize said radiation generated from saidreaction between said first and second gas samples.
 3. The apparatus ofclaim 2, wherein a reaction between said first and second gas samples isperformed in a pressure range of about 0.1 to about 50 mbar.
 4. Theapparatus of claim 2, wherein said optical detection system comprises anoptical device which selectively transmits radiation originating fromsaid reaction between said first and second gas samples and aphotodetector capable of detecting said transmitted radiation.
 5. Theapparatus of claim 4, wherein said optical device transmits radiationwith a full-width half-maximum bandpass of between about 331 nm andabout 341 nm.
 6. The apparatus of claim 1, wherein said first gas samplecomprises ammonia.
 7. The apparatus of claim 5, wherein said radiationarising from said first gas sample originates from NH* radical.
 8. Theapparatus of claim 6, wherein said concentration of ammonia in saidfirst gas sample is determined by comparing said radiation intensityparameter with at least one provided set of calibration data regardingammoniacal species concentration or a functional relationshipcorrelating detected radiation from excited imidogen radicals withammoniacal species concentration.
 9. The apparatus of claim 8, whereinsaid calibration procedure is performed inside said reactor by reactingeach one of a plurality of calibration gas samples comprising knownconcentrations of ammonia with a reactant gas comprising fluorine,detecting radiation from NH* radicals generated from each reaction,converting said detected radiation into a calibrated radiation intensityparameter for each of said plurality of calibration gas samples, andforming calibration data associating said calibrated radiation intensityparameter with its corresponding said known concentration of ammonia.